1. Field of the Invention
This invention relates to ethylene polymer based compositions which are useful as primary insulation for copper conductors which are used in wire and cable which contain waterproofing filler and which can be used under elevated temperature conditions.
2. Description of the Prior Art
The current trend in the installation of telephone wire and cable which contains copper conductors is to install the wire or cable underground, in a duct, or by direct burial. To protect the wire or cable from water damage under such conditions, it is filled with waterproofing fillers. Filled cables of this type are disclosed in U.S. Pat. No. 3,668,298. The use of the waterproofing fillers in such cables, however, presents compatability problems with respect to the polyolefin resin based compositions which have been used to date as the primary insulation for such wire and cable, and which primary insulation comes into contact with the filler during the use of the wire and cable. If the polyolefin resin and the filler are too compatible the resin will absorb the filler and swell and possibly also leach out from the insulation, stabilizers and other components of the insulation composition.
The selection of suitable synthetic resins for use as primary insulation in the filled wire or cable is further complicated by the fact that the resin based composition which must be used in this regard must be stable with the filler at elevated temperatures of the order of about 40.degree. to 70.degree. C. for extended periods of time, and the resin must also be stable during extrusion thereof around the individual copper conductors at elevated temperatures of the order of about 200.degree. to 300.degree. C.
Although the filled wire or cable is installed underground where temperatures rarely reach a level which is likely to significantly hasten the degradation of the insulation, there are portions of the wire or cable which will experience a more hostile environment. This occurs when splices or tie-ins are made aboveground and the filler is removed from around a section of the conductor, and the insulated conductors are then left exposed to the air. The exposed conductors are usually left in an enclosed box or pedestal in which temperatures approaching 70.degree. C may be reached in the field. The aging of the insulation in this environment represents the most crucial test of insulation stability.
The extrusion of the primary insulation, and the use of the extruded insulation, under significantly different environmental conditions than those previously encountered also requires the use of antioxidants which will be effective under such conditions for the protection of the insulation against oxidative degradation. The antioxidants to be used, however, must also be chemically and physically compatible with the filler, and the other components of the insulation composition, and the copper conductor itself.
Thus, the other components of the insulation compositions must also include copper deactivators to counteract the tendency of the copper conductor to catalytically influence the oxidative degradation of the synthetic resin in the primary insulation and any absorbed filler.
Thus, the formulation of a primary insulation composition which will be useful in contact with copper conductors and waterproofing fillers under elevated use temperatures, and after being subjected to elevated extrusion temperatures, requires the use of a unique combination of materials, which combinations are not readily achieved.